The asynchronous transfer mode (ATM) environment is now widely recognized as the preferred way of implementing Broadband Integrated Services Digital Network (B-ISDN) multiservice networks for simultaneously carrying voice, data, facsimile, and video on the network. ATM networks transmit signals in short, fixed-size packets of information. ATM networks, by virtue of being packet-based, can exploit the bursty nature of voice, data, facsimile, and video to multiplex packets from many sources so that transmission bandwidth and switching resources are efficiently shared.
The initial implementation of ATM networks will likely take the form of small, local networks or "islands." In each local ATM network, it is expected that bandwidth will be relatively abundant. Packet losses and bit errors due to traffic congestion will be rare because some form of congestion control will be in effect. Voice-band signals will likely be transported on ATM networks as a continuous stream of pulse code modulated bits (PCM) at 64 kilobits per second. No digital speech interpolation will be used to remove periods of silence in the voice-band signal. Compression of voice-band signals would also be unnecessary under the PCM transport scheme because of the minimal bandwidth limitations.
Since it is unlikely that ATM can be implemented in all locations simultaneously, it would be desirable to interconnect the various local ATM networks, located in different geographic regions and in different countries, using existing telephone networks. In most existing public switched telephone networks (PSTNs), bit errors and congestion are common due to bandwidth limitations on the PSTN. Unfortunately, the combination of congestion control, PCM transport, and continuous bitstream means that ATM networks cannot be efficiently implemented in a bandwidth-limited environment. Therefore, to achieve connection between an ATM network and a PSTN, some way of optimizing bandwidth usage between these networks would be desirable.